Chapter 3

BIOMASS ENERGY – GASIFICATION TECHNOLOGY

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3.1 Potential of Biomass

Biomass can be defined as solar energy stored in the form of organic

matter like plants or animal residues that have a net positive value as a

chemical resource.

India, being agriculture based country is having high potential for

biomass. The common biomass abundantly available in rural India is

agricultural residues like rice husk, groundnut shell, coconut shell, bagasse,

corn cobs, rice straw, cotton stalks, animal dung, forest wastes like bark,

chips, saw dust, fire wood etc. India produces a huge quantity of agricultural

residues, which can be converted into energy. The age-old practice of direct

burning of agricultural residues to generate energy for domestic as well as

industrial applications is very inefficient. Amongst the different technologies

available for energy conversion of biomass like biomass briquitting, biomass

combustion, bio-methanation, biomass gasification, biomass cogeneration,

biomass digestion etc., Biomass gasification is one of the efficient and useful

technologies.

Biomass gasification means incomplete combustion of biomass resulting

in production of combustible gases consisting of Carbon monoxide (CO),

Hydrogen (H2) and traces of Methane (CH4). This mixture is called producer

gas. Producer gas can be used to run internal combustion engines (both

compression and spark ignition), it can be used as substitute for furnace oil in

direct heat applications and can be used to produce, in an economically viable

way, methanol – an extremely attractive chemical which is useful both as fuel

for heat engines as well as chemical feedstock for industries. Since any

biomass material can undergo gasification, this process is much more

attractive than ethanol production or biogas where only selected biomass

materials can produce the fuel.

Fig: 3.1 Use of Biomass for energy

Besides, there is a problem that solid wastes (available on the farm) are

seldom in a form that can be readily utilized economically. For example wood

wastes can be used in hog fuel boiler but the equipment is expensive and

energy recovery is low. As a result it is often advantageous to convert this

waste into more readily usable fuel form like producer gas hence, the

attractiveness of gasification.

However under present conditions, economic factors seem to provide the

strongest argument of considering gasification. In many situations where the

price of petroleum fuels is high or where supplies are unreliable the biomass

gasification can provide an economically viable system – provided the suitable

biomass feedstock is easily available.

3.2 Historical Background

The process of gasification to produce combustible from organic feeds was

used in blast furnaces over 180 years ago. The possibility of using this gas for

heating and power generation was soon realized and there emerged in Europe

producer gas systems, which used charcoal and peat as feed material. At the

turn of the century, petroleum gained wider use as a fuel, but during both

world wars and particularly World War II, shortage in petroleum supplies led to

widespread re-introduction of gasification. By 1945 the gas was being used to

power trucks, buses and agricultural and industrial machines. It is estimated

that there were close to 9000,000 vehicles running on producer gas all over the

world.

After World War II the lack of strategic impetus and the availability of cheap

fossil fuels led to general decline in the producer gas industry. However Sweden

continued to work on producer gas technology and the work was accelerated

after 1956 Suez Canal crisis. A decision was then made to include gasifiers in

Swedish strategic emergency plans. Research into suitable designs of wood

gasifiers, essentially for transport use, was carried out at the National Swedish

Institute for Agricultural Machinery Testing and is still in progress.

The contemporary interest in small scale gasifier R&D, for most part dates

from 1973 oil crisis. The manufacturing also took off with increased interest

shown in gasification technology. At present there are about 64 gasification

equipment manufacturers all over the world.

3.3 Producer gas fuel

Producer gas derived from biomass, typically contains 18-20% each of H2

and CO, 2% CH4 and rest inert gases like CO2 and N2. The lower calorific value

varies between 4.5 – 4.9 MJ/kg, with stochiometric air-to-fuel ratio being 1.25

+ 0.05 on mass basis. Some of the fundamental data relating to producer gas

are compared with pure gases in Appendix VI. The comparison of producer gas

with methane is more vital with regard to the internal combustion engine

operation. This is because most of the engines operating on gaseous fuels are

either close to pure methane (natural gas) or diluted methane (bio-gas, land-fill

gas). The fuel-air equivalence ratio (actual fuel to air ratio)/(stoichiometric fuel

to air ratio) at the flammability limits compares closely for both the gases, but

the laminar burning velocity for producer gas at the lean limits is much higher.

The laminar burning velocity for producer gas (at 0.1MPa, 300K) is about 0.5

m/sec, which is about 30% higher than methane. This feature is argued to

demand lower advancement in the ignition timing and needs consideration

while arriving at the optimum ignition timing for the producer gas fuel.

Like any other gaseous fuel, producer gas can be used for internal

combustion engine operation provided the gas is sufficiently clean such that

contaminant does not accumulate in the intermediary passages to the engine

cylinder. But this fuel has largely been left unexploited due to additional

perceptions, namely

auto-ignition tendency at higher compression ratio,

Large de-rating in power due to energy density being low.

However, these perceptions need re-examination and clarification. The

arguments against the classical view in favor of better knock resistivity are as

follows. Firstly, with the laminar burning velocity being high due to the

presence of hydrogen might reduce the tendency for the knock. Secondly, the

presence of inert in the raw gas (CO2 and N2) might suppress the pre-flame

reactions that are responsible for knocking on account of increased dilution.

Also the maximum flame temperature attainable with the producer gas being

lower compared to conventional fuels like methane, one could expect better

knock resistivity.

Further, there is a general perception that producer gas being a low-

density energy fuel, the extent of de-rating in power would be large when

compared to high energy density fuels like Natural Gas and Liquefied

Petroleum Gas. This could be misleading because what needs to be accounted

for comparison is the mixture energy density and not the fuel energy density.

On comparison with CH4, the mixture energy density for producer gas is lower

by 23% as reflected in Appendix VI. The product to reactant mole ratio for

producer gas is less than one. These two parameters could contribute to de-

rating of engine output. However, it might be possible to reduce de-rating by

working with engines of higher compression ratio, perhaps higher than what

has been examined using natural gas (CR=15.8).

3.4 Theory Of Gasification

The production of generator gas (producer gas) called gasification, is partial

combustion of solid fuel (biomass) and takes place at temperatures of about

10000C. The reactor is called a gasifier. The combustion products from

complete combustion of biomass generally contain nitrogen, water vapor,

carbon dioxide and surplus of oxygen. However in gasification where there is a

surplus of solid fuel (incomplete combustion) the products of combustion are

(Figure 3.2) combustible gases like Carbon monoxide (CO), Hydrogen (H2) and

traces of Methane and non useful products like tar and dust. The production of

these gases is by reaction of water vapor and carbon dioxide through a glowing

layer of charcoal. Thus the key to gasifier design is to create conditions such

that biomass is reduced to charcoal and, charcoal is converted at suitable

temperature to produce CO and H2.

Co

H2

CH4

Tar

Dust

Gasifier

Fig: 3.2 Products of Gasifier

3.5 Types of Gasifiers

Since there is an interaction of air or oxygen and biomass in the gasifier,

they are classified according to the way air or oxygen is introduced in it. There

are three types of gasifiers Downdraft, Updraft and Cross draft as shown in

Figure 3.3. And as the classification implies updraft gasifier has air passing

through the biomass from bottom and the combustible gases come out from

the top of the gasifier. Similarly in the downdraft gasifier the air is passed from

Any Combustible biomass material

Air

the tuyers in the downdraft direction. With slight variation almost all the

gasifiers fall in the above categories.

Fig. 3.3. Various types of gasifiers

The choice of one type of gasifier over other is dictated by the fuel, its

final available form, its size, moisture content and ash content. Table 3.1 lists

therefore, the advantages and disadvantages generally found for various

classes of gasifiers.

Table 3.1 Advantages and Disadvantages of various Gasifiers

Sr. No. Gasifier

Type

Advantage Disadvantages

1. Updraft - Small pressure drop

- good thermal efficiency

- little tendency towards slag

- Great sensitivity to

tar and moisture

and moisture

formation content of fuel

- relatively long time

required for start

up of IC engine

- poor reaction

capability with

heavy gas load

2. Downdraft - Flexible adaptation of gas

production to load

- low sensitivity to charcoal

dust and tar content of

fuel

- Design tends to be

tall

not feasible for very

small particle size of

fuel

3. Cross draft - Short design height

- very fast response time to

load

- flexible gas production

Very high sensitivity

to slag formation

- high pressure drop

3.6 Process Zones

Four distinct processes take place in a gasifier as the fuel makes its way to

gasification. They are:

Drying of fuel

Pyrolysis – a process in which tar and other volatiles are driven off

Combustion

Reduction

Though there is a considerable overlap of the processes, each can be

assumed to occupy a separate zone where fundamentally different chemical

and thermal reactions take place. Figure

their respective temperatures in the

gasifiers respectively.

Fig. 3.4 Various zones in downdraft gasifier

a process in which tar and other volatiles are driven off

Though there is a considerable overlap of the processes, each can be

assumed to occupy a separate zone where fundamentally different chemical

and thermal reactions take place. Figure 3.4 and 3.5 show different zones and

their respective temperatures in the regions for downdraft and cross

Fig. 3.4 Various zones in downdraft gasifier

a process in which tar and other volatiles are driven off

Though there is a considerable overlap of the processes, each can be

assumed to occupy a separate zone where fundamentally different chemical

different zones and

regions for downdraft and cross draft

Fig. 3.4 Various zones in downdraft gasifier

Fig. 3.5. Various zones of cross draft gasifier

In the downdraft gasifiers there are two types

throat gasifiers as shown in figure 3.6.

Fig. 3.6

Fig. 3.5. Various zones of cross draft gasifier

In the downdraft gasifiers there are two types viz., Single throat and Double

gasifiers as shown in figure 3.6.

Fig. 3.6 Single and double throat gasifiers

Single throat and Double

Single and double throat gasifiers

Single throat gasifiers are mainly used for stationary applications whereas

double throat are for varying loads as well as automotive purposes.

3.7 Reaction Chemistry

The following major reactions take place in combustion and reduction zone.

3.7.1 Combustion zone

The combustible substance of a solid fuel is usually composed of

elements carbon, hydrogen and oxygen. In complete combustion carbon dioxide

is obtained from carbon in fuel and water is obtained from the hydrogen,

usually as steam. The combustion reaction is exothermic and yields a

theoretical oxidation temperature of 14500C. The main reactions are:

C + O2 = CO2 (+ 393 MJ/kg mole) (1)

2H2 + O2 = 2H2 O (- 242 MJ/kg mole) (2)

3.7.2 Reaction zone

The products of partial combustion (water, carbon dioxide and

uncombusted partially cracked pyrolysis products) now pass through a red-hot

charcoal bed where the following reduction reactions take place.

C + CO2 = 2CO (- 164.9 MJ/kg mole) (3)

C + H2O = CO + H2 (- 122.6 MJ/kg mole) (4)

CO + H2O = CO + H2 (+ 42 MJ/kg mole) (5)

C + 2H2 = CH4 (+ 75 MJ/kg mole) (6)

CO2 + H2 = CO + H2O (- 42.3 MJ/kg mole) (7)

Reactions (3) and (4) are main reduction reactions and being endothermic

have the capability of reducing gas temperature. Consequently the

temperatures in the reduction zone are normally 800-10000C. Lower the

reduction zone temperature (~ 700-8000C), lower is the calorific value of gas.

3.7.3 Pyrolysis zone

Wood pyrolysis is an intricate process that is still not completely

understood. The products depend upon temperature, pressure, residence time

and heat losses. However following general remarks can be made about them.

Up to the temperature of 2000C only water is driven off. Between 200 to

2800C carbon dioxide, acetic acid and water are given off. The real pyrolysis,

which takes place between 280 to 5000C, produces large quantities of tar and

gases containing carbon dioxide. Besides light tars, some methyl alcohol is also

formed. Between 500 to 7000C the gas production is small and contains

hydrogen. Thus it is easy to see that updraft gasifier will produce much more

tar than downdraft one. In downdraft gasifier the tars have to go through

combustion and reduction zone and are partially broken down. Since majority

of fuels like wood and biomass residue do have large quantities of tar,

downdraft gasifier is preferred over others. Indeed majority of gasifiers, both in

World War II and presently are of downdraft type.

Finally in the drying zone the main process is of drying of wood. Wood

entering the gasifier has moisture content of 10-30%. Various experiments on

different gasifiers in different conditions have shown that on an average the

condensate formed is 6-10% of the weight of gasified wood. Some organic acids

also come out during the drying process. These acids give rise to corrosion of

gasifiers.

3.8 Properties of Producer gas

The producer gas is affected by various processes as outlined above hence

one can expect variations in the gas produced from various biomass sources.

The gas composition is also a function of gasifier design and thus, the same

fuel may give different calorific value as when used in two different gasifiers.

Table 3.2 shows the composition of gas produced from various sources and

approximate values of gas from different fuels.

FuelGasificatio

n method

Volume percentagesCalorific value

MJ/m3

CO H2 CH4 CO2 N2

Charcoal Downdraft28-

315-10 1-2 1-2 55-60 4.60-5.65

Wood with 12-20%

moisture contentDowndraft

17-

22

16-

202-3

10-

1555-50 5.00-5.86

Wheat straw pellets Downdraft 14- 17- - 11- - 4.50

Table 3.2 Composition of Producer Gas from various fuels

The maximum dilution of gas takes place because of presence of nitrogen.

Almost 50-60% of gas is composed of noncombustible nitrogen. Thus it may be

beneficial to use oxygen instead of air for gasification. However the cost and

availability of oxygen may be a limiting factor in this regard. Nevertheless

where the end product is methanol – a high energy quality item, then the cost

and use of oxygen can be justified.

3.8.1 Temperature of Gas

17 19 14

Coconut husks Downdraft16-

20

17-

19.5-

10-

15- 5.80

Coconut shells Downdraft19-

24

10-

15-

11-

15- 7.20

Pressed Sugarcane Downdraft15-

18

15-

18-

12-

14- 5.30

Charcoal Updraft 30 19.7 - 3.6 46 5.98

Corn cobs Downdraft 18.6 16.5 6.4 - - 6.29

Rice hulls pelleted Downdraft 16.1 9.6 0.95 - - 3.25

Cotton stalks

cubedDowndraft 15.7 11.7 3.4 - - 4.32

On an average the temperature of gas leaving the gasifier is about 300 to

4000C. If the temperature is higher than 5000C, it is an indication that partial

combustion of gas is taking place. This generally happens when the air flow

rate through the gasifier is higher than the design value.

3.9 Gasifier Fuel Characteristics

Almost any carbonaceous or biomass fuel can be gasified under

experimental or laboratory conditions. However the real test for a good gasifier

is not whether a combustible gas can be generated by burning a biomass fuel

with 20-40% stoichiometric air but that a reliable gas producer can be made

which can also be economically attractive to the customer. Towards this goal

the fuel characteristics have to be evaluated and fuel processing to be done.

Many of gasifier manufacturers claim that a gasifier is available which can

gasify any fuel but there is no such thing as a universal gasifier. A gasifier is

very fuel specific and it is tailored around a fuel rather than the other way

round.

Thus a gasifier fuel can be classified as good or bad according to the

following parameters:

Energy content of the fuel

Bulk density

Moisture content

Dust content

Tar content

Ash and slagging characteristic

3.9.1 Energy content and Bulk Density of fuel

The higher the energy content and bulk density of fuel, the similar is the

gasifier volume since for one charge one can get power for longer time.

3.9.2 Moisture content

In most fuels there is very little choice in moisture content since it is

determined by the type of fuel, its origin and treatment. It is desirable to use

fuel with low moisture content because heat loss due to its evaporation before

gasification is considerable and the heat budget of the gasification reaction is

impaired. For example, for fuel at 250C and raw gas exit temperature from

gasifier at 3000C, 2875 KJ/kg heat must be supplied by fuel to heat and

evaporate moisture.

Besides impairing the gasifier heat budget, high moisture content also

puts load on cooling and filtering equipment by increasing the pressure drop

across these units because of condensing liquid. Thus in order to reduce the

moisture content of fuel some pretreatment of fuel is required. Generally

desirable moisture content for fuel should be less than 20%.

3.9.3 Dust content

All gasifier fuels produce dust. This dust is a nuisance since it can clog

the internal combustion engine and hence has to be removed. The gasifier

design should be such that it should not produce more t

Figure 3.7 shows dust produced as a function of gas production for wo

generators used during World War II.

The higher the dust produced, more load is put on filters necessitating their

frequent flushing and increased maintenance.

Fig. 3.7 Dust content as a function of gas production

3.9.4 Tar content

Tar is one of the most unpleasant constituents of the gas as it tends to

deposit in the carburetor and intake valves causing sticking and troublesome

operations. It is a product of highly ir

Pyrolysis zone. The physical property of tar depends upon temperature and

heat rate and the appearance ranges from brown and watery (60% water) to

black and highly viscous (7% water). There are approximately 200 chemi

constituents that have been identified in tar so far.

design should be such that it should not produce more than 2-6 g/m

7 shows dust produced as a function of gas production for wo

generators used during World War II.

The higher the dust produced, more load is put on filters necessitating their

frequent flushing and increased maintenance.

Dust content as a function of gas production

Tar is one of the most unpleasant constituents of the gas as it tends to

deposit in the carburetor and intake valves causing sticking and troublesome

operations. It is a product of highly irreversible process taking place in the

Pyrolysis zone. The physical property of tar depends upon temperature and

heat rate and the appearance ranges from brown and watery (60% water) to

black and highly viscous (7% water). There are approximately 200 chemi

constituents that have been identified in tar so far.

6 g/m3 of dust.

7 shows dust produced as a function of gas production for wood

The higher the dust produced, more load is put on filters necessitating their

Dust content as a function of gas production

Tar is one of the most unpleasant constituents of the gas as it tends to

deposit in the carburetor and intake valves causing sticking and troublesome

reversible process taking place in the

Pyrolysis zone. The physical property of tar depends upon temperature and

heat rate and the appearance ranges from brown and watery (60% water) to

black and highly viscous (7% water). There are approximately 200 chemical

Very little research work has been done in the area of removing or

burning tar in the gasifier so that relatively tar free gas comes out. Thus the

major effort has been devoted to cleaning this tar by filters and coolers. A well-

designed gasifier should put out less than 1 g/m3 of tar. Usually it is assumed

that a downdraft gasifier produces less tar than other gasifiers. However

because of localized inefficient processes taking place in the throat of the

downdraft gasifier it does not allow the complete dissociation of tar. More

research effort is therefore needed in exploring the mechanism of tar

breakdown in downdraft gasifiers.

3.9.5 Ash and slagging characteristics

The mineral content in the fuel that remains in oxidized form after

complete combustion is usually called ash. The ash content of a fuel and the

ash composition has a major impact on trouble free operation of gasifier.

Ash basically interferes with gasification process in two ways:

It fuses together to form slag and this clinker stops or inhibits the

downward flow of biomass feed.

Even if it does not fuse together it shelters the points in fuel where

ignition is initiated and thus lowers the fuel’s reaction response.

Ash and tar removal are the two most important processes in gasification

system for its smooth running. Various systems have been devised for ash

removal. In fact some fuels with high ash content can be easily gasified if

elaborate ash removal system is installed in the gasifier.

Slagging, however, can be overcome by two types of operation of gasifier:

Low temperature operation that keeps the temperature well below

the flow temperature of the ash.

High temperature operation that keeps the temperature above the

melting point of ash.

The first method is usually accomplished by steam or water injection

while the latter method requires provisions for tapping the molten slag out of

the oxidation zone. Each method has its advantages and disadvantages and

depends on specific fuel and gasifier design.

Charcoal, specifically, because of being tar free and having relatively low

ash content property was the preferred fuel during World War II and still

remains so. However there is a major disadvantage of charcoal in terms of

energy. Charcoal is mostly produced from wood and in conversion of wood to

charcoal about 50% of original energy is lost. When made by pit method the

losses can be as high as 80%. Besides with the present energy crisis where

most countries do not have enough supply of wood it is advantageous and

attractive to use agricultural residues. For the agricultural sector this is an

extremely attractive alternative.

Many agricultural residues and fuels have, therefore, been gasified.

However the operating experience is very limited and most of the work has

been on laboratory scale. More research needs to be done in order to make

gasification systems running on these fuels on a large scale.

3.10 Gasification Systems

The combustible gases from the gasifier can be used

In internal combustion engines,

For direct heat applications and

As feedstock for production of chemicals like methanol.

3.10.1 Shaft power systems

The biggest application of producer gas has been in driving IC engines.

Both spark ignition and compression ignition engines have been driven by it.

In principle any IC engine can be converted to run completely or partly on the

producer gas. However in actual practice running the engines uninterrupted

and for long periods of time without any problem is difficult to achieve.

Producer gas being a relatively low energy gas has certain combustion

characteristics that differ markedly from gasoline or diesel oil. Thus in future

R&D in gasification it is worthwhile to do considerable work to make an engine

specific for the gas.

3.10.1.1 Spark Ignition Engine

When a spark ignition engine is converted to operation on producer gas it

is derated to about 40-50%. The deration is primarily because of low energy

density of producer gas. This accounts for about 30% loss of power. The rest is

accounted for by the pressure drop in the intake valves and piping. A spark

ignition engine on the whole requires very little modification to run on

producer gas. Generally depending upon the maker of engine (compression

ratio and rpm), the ignition timing has to be advanced by about 30-40 degrees.

This is done because of low flame speed of producer gas as compared to

gasoline. The low flame speed of producer gas is more efficiently used in low

speed engine. Thus an engine with 1500 – 2500 rpm is ideal for producer gas

application.

It should be noted that in general the overall efficiency of the IC engine

itself does not change though the power derating takes place. However,

detailed comparison of the engine efficiencies with and without producer gas

has not been done till now because of insufficient data and large variations in

producer gas composition. Thus a conservative figure of 15 – 20% can be used

efficiently in spark ignition engines.

With the above efficiencies it is easy to calculate the mechanical energy

available per kg of biomass gasified. With gasifier efficiency of 68% the total

system efficiency (gasification and engine) is 10-13%. Thus on an average one

can get 0.55-0.75 kWh of mechanical energy per kilogram of biomass gasified

(calorific value of biomass is taken to be 19.6 MJ/kg. This value however

changes with load and can go as low as 0.22 kWh/kg for 10% load to 0.73

kWh/kg for 87% load rated capacity. Nevertheless for purpose of sizing a

system, a good number is 0.7 kWh/kg.

3.10.1.2 Compression Ignition Engine

A compression ignition or diesel engine cannot be operated on producer gas

completely without injection of small amount of diesel. This is because the

producer gas cannot ignite by itself under prevailing pressure. Thus for

compression ignition engines to run on producer gas they have to be either

A dual fuel engine, or

Converted into spark ignition engines.

Since diesel engines have compression ratio between 16-20 and are run at

lower rpm than gasoline engines they are ideally suited to run on producer

gases with spark ignition. However, conversion of the engine to spark ignition

is costly and elaborate affair and the advantages are nullified by the cost. Thus

most of the diesel engines running on producer gas have been dual fuel type.

Especially in developing countries, where the proliferation of diesel engine has

been because of dual pricing structure (diesel is subsidized).

Because of high compression ratio and low speeds, the derating of diesel

engines running on producer gas is only between 15-30%. This is far superior

to the gasoline engine’s derating. Even if gasoline engines are used in dual fuel

mode their derating is still between 40-50%.

On an average the diesel engine can run on 15-20% (of the original

consumption) diesel and rest on producer gas. Generally the engine is started

on diesel and as the gas generation builds up the diesel consumption is then

kept at idling level. The engine efficiency in this case is about 25%. Thus as a

thumb rule the dual fuel engine producing 1 kWh requires 1 Kg of biomass and

consumes 0.07 liters of diesel. In both the diesel and gasoline engines the

introduction of producer gas to the engine is by a T valve where, from one

section of the T air is sucked in from one section producer gas sucked in and

one is connected to inlet manifold of engine, thus the complicated carburetor is

greatly simplified by the above arrangements. Many arrangements have been

developed for introduction of air/gas mixture in the engine.

3.10.2 Direct Heat Systems

Direct heat systems are those in which the producer gas is burnt directly

in furnace or boiler. The advantage of this as compared to direct combustion of

biomass is in obtaining controlled heating and higher flame temperatures than

those obtainable otherwise. Because of direct burning, the gas quality is less

critical than in shaft power systems and consequently they are less demanding

on cooling and cleaning equipment and have more versatility as far as fuels are

concerned.

The direct heat systems have great attraction for agricultural

applications like drying of farm produce and consequently many such

applications are under way in U.S. Since direct heat systems were rarely used

during Second World War, their experience is recent.

3.10.3 Fluidized Bed Systems

For fuels which have high ash content and the ash has low melting point,

fluidized bed combustion seems to gasify them. In fluidized bed gasifiers the air

is blown upwards through the biomass bed. The bed under such conditions

behaves like boiling fluid and has excellent temperature uniformity and

provides efficient contact between gaseous and solid phase. Generally the heat

is transferred initially by hot bed of sand. The major advantage of fluidized bed

gasifier over, say, downdraft is its flexibility with regard to feed rate and its

composition. Fluidized bed systems can also have high volumetric capacity and

the temperature can be easily controlled. However these advantages are offset

by the complexity of the system with large blowers for blowing air and augers

for feeding biomass. Besides, fluidized bed systems produce more dust and tar

as compared to downdraft gasifiers. This puts a heavy load on the cooling and

cleaning train.

Nevertheless quite a number of research projects are under way to study

and optimize biomass residue based fluidized bed gasification systems.

However no large scale manufacturing facilities for such systems exists today.

3.11 Cooling and Cleaning of Gas

However for all these applications the most important ingredient is the

availability of biomass fuel. For on farm applications biomass residues are

attractive propositions. However, before any large scale application of

gasification is undertaken, the fuel availability is to be critically ascertained. If

the biomass residue availability is not adequate then the decision has to be

made about running it on wood. However such a decision can only be made at

specific sites and for specific applications. Just like in any other alternative

energy source it is advisable to use hybrid systems, similarly in biomass

gasification systems also it will be worthwhile to use them in conjunction with

other energy systems. For example, grain drying can have biomass

gasifier/solar coupling.

However in order to utilize the producer gas for any of the above

applications it should be cleaned of tar and dust and be cooled. So cooling and

cleaning of the gas is one of the most important processes in the whole

gasification system. The failure or the success of producer gas units depends

completely on their ability to provide a clean and cool gas to the engines or for

burners. Thus the importance of cleaning and cooling systems cannot be

ignored.

The temperature of gas coming out of generator is normally between 300

0C and 500 0C. This gas has to be cooled in order to raise its energy density.

Various types of cooling equipment have been used to achieve this end. Most

coolers are gas to air heat exchangers where the cooling is done by free

convection of air on the outside surface of heat exchanger. Since the gas also

contains moisture and tar, some heat exchangers provide partial scrubbing of

gas. Thus ideally the gas going to an internal combustion engine should be

cooled to nearly ambient temperature.

Cleaning of the gas is trickier and is very critical. Normally three types of

filters are used in this process. They are classified as dry, moist and wet.

In the dry category are cyclone filters. They are designed according to the

rate of gas production and its dust content. The cyclone filters are useful for

particle size of 5 μm and greater. Since 60-65% of the producer gas contains

particles above 60 μm in size the cyclone filter is an excellent cleaning device.

After passing through cyclone filter the gas still contains fine dust,

particles and tar and it is further cleaned by passing through either a wet

scrubber or dry cloth filter. In the wet scrubber the gas is washed by water in

counter-current mode. The scrubber also acts like a cooler, from where the gas

goes to cloth or cork filter for final cleaning.

Since cloth filter is a fine filter, any condensation of water on it stops the

gas flow because of increase in pressure drop across it. Thus in quite a

number of gasification systems the hot gases are passed through the cloth

filter and then only do they go to the cooler. Since the gases are still above dew

point, no condensation takes place in filter.

There is quite a substantial pressure drop across the whole gasification

system and the design is usually done such that the pressure drop should not

exceed 100 cm of water.

3.12 Hazards Associated with the use of Producer Gas

During the gasifier operation process one can encounter with various

dangers namely toxic, fire and explosion hazards. Moreover, this process and

men’s activities connected with it can affect the environment. Thus, it should

consider the environmental hazards of the gasifier operation. A review of the

different types of hazards and environmental impacts of producer gas operation

are presented in this section.

3.12.1 Toxic hazards

An important component of the producer gas is carbon monoxide (CO) –

an extremely toxic and dangerous gas as it tends to combine with the

hemoglobin of the blood and thus, prevents oxygen absorption and

distribution. The carbon monoxide is also extremely dangerous, because it is

odorless and tasteless. Generally, producer gas system works under suction, so

that even if a minor leakage occurs, no dangerous gases will escape from the

equipment during actual operation, thus the risk of poisoning through leakage

is minimal. The situation is, however, different during starting-up and

shutting-down of the gasification installation. When the engine is shut down,

formation of the producer gas still continues, resulting in pressure increasing

inside the gasifier. This pressure increase lasts for approximately 20 minutes

after the engine shutdown. Thus, it is not advisable to stay in the vehicle

during this period. Moreover, the gasification system should be allowed to cool

for at least 20 minutes before the vehicle is placed in the closed garage,

especially, connected with the living premises. The gas formed during the

shutdown period has a carbon monoxide 53 content of 23 up to 27% and is,

thus, very toxic. It is still arguing, whether chronic poisoning can occur as a

result of prolonged inhalation of relatively small amounts of carbon monoxide.

But, to the present moment the majority of researchers tend to opinion that no

chronic symptoms can occur through carbon monoxide poisoning.

3.12.2 Fire hazards

Fire hazards can result from the following causes:

- high surface temperature of equipment;

- risks of sparks during refueling;

- flames through gasifier air inlet on refueling lid (may ignite

near by flammable materials).

However, fire risks can be considerably decreased by taking certain

precautions. First of all, hot parts of the system must be carefully insulated.

Secondly, installation of the double-sluice type filling device and back-firing

valve at the gasifier inlet should be considered. Moreover, size of the clearance

between the gasifier unit and the vehicle should be carefully chosen. Also the

ash disposal should be carried out only after the unit cool down (this residue

must be placed away from any combustible material).

3.12.3 Explosion hazards

Risk of explosion can arise from the producer gas-air explosive mixture

formation. This mixture can be formed under the following circumstances:

- air leakage into the gas system;

- air penetration during refueling process;

- air leakage into the cold gasifier still containing residual producer

gas, which can ignite;

- back-firing from the fan exhaust burner when the system is filled

with a combustible mixture of air and gas during the starting-up

process.

Air leakage into the gas system does not generally give rise to explosions. If a

leakage occurs in the lower part of the gasifier, this will result in partial

combustion of the gas, which in turn will lead to higher gas outlet

temperatures and a lower gas quality.

Air leakage into the cold gasifier and residual producer gas-air mixture

ignition will lead to an explosion. That is why, the cold system should always

be carefully ventilated before igniting to ensure that it contains only fresh air.

3.12.4 Environmental hazards

During the gasification process of wood or agricultural residues, besides

the producer gas itself, certain amounts of ash (from the gasifier unit and the

gas cleaning system) and condensate, polluted by phenol and tar, are

produced.

The ash disposal creates no problem, but with the tarry condensate the

situation is quite different – disposal of this condensate from a large number of

the gasifiers can have undesirable environmental effects. Thus, this problem

(the tarry condensate disposal) needs careful investigation.

There is one more question to consider, when talking about the wide use of the

biomass gasification, the increased demand for wood and agricultural residues

fuels. The effect of this increased demand on the environment can be either

positive or negative, depending on the approach to the problem. Selective wood

cutting and use of some residues can improve the forests and fields.

Indiscriminate cutting and use of residues can lead to poor wood lot stands,

erosion and soil depletion.